The population of the three RSB species namely, white stem borer (WSB), Scirpophaga innotata (Walker), striped stem borer (SSB), Chilo suppressalis (Walker) and pink stem borer (PSB), Sesamia inference (Walker) was monitored in a five ha irrigated lowland rice field planted with medium maturing rice variety IR 77186-122-2-2-3 (NSIC 158) during the 2015 wet season cropping. Field sampling was done at 21 days and then at fortnightly interval thereafter. A total of 100 hills with RSB damage were collected per sampling. Infested tillers were dissected and the number of RSB larvae present was recorded. WSB and SSB were present throughout the crop growing season, while PSB was only recorded during the flowering stage. SSB was the most dominant species followed by WSB and PSB. Interestingly, co-occurrence of RSB species in a single tiller was prevalent from seedling to maturity. At vegetative stage, 52% of the “deadheart” was attributed to combined effects of WSB+SSB larvae feeding. At reproductive stage, 44% of the “whitehead” was due to WSB+SSB and WSB+PSB. The extent of damage posed by mutualistic association of two RSB species is much greater than the average individual effects. There were no records of SSB and PSB occupying the same tiller. This complex population dynamics exhibited by RSB species and the extent of crop losses accounted to RSB infestation should be considered in rice research and development programs in the Philippines.